1. Technical Field
The present invention relates generally to high-performance, fault-tolerant content delivery in a content delivery network (CDN).
2. Description of the Related Art
Streaming media is a type of Internet content that has the important characteristic of being able to be played while still in the process of being downloaded. A client can play the first packet of the stream, and decompress the second, while receiving the third. Thus, an end user can start enjoying the multimedia without waiting to the end of transmission. Streaming is very useful for delivering media because media files tend to be large particularly as the duration of the programming increases. Indeed, for live events, the file size is, in effect, infinite. To view a media file that is not streamed, users must first download the file to a local hard disk—which may take minutes or even hours—and then open the file with player software that is compatible with the file format. To view streaming media, the user's browser opens player software, which buffers the file for a few seconds and then plays the file while simultaneously downloading it. Unlike software downloads, most streaming media files are not stored locally on a user's hard disk. Once the bits representing content are used, the player typically discards them.
Streaming media quality varies widely according to the type of media being delivered, the speed of the user's Internet connection, network conditions, the bit rate at which the content is encoded, and the format used. In general, streaming audio can be FM quality, but, given typical bandwidth constraints, streaming video is poor by TV standards, with smaller screens, lower resolution, and fewer frames per second. The source for streaming media can be just about any form of media, including VHS or Beta format tapes, audio cassettes, DAT, MPEG video, MP3 audio, AVI, and the like. Prior to streaming, the content must first be encoded, a process which accomplishes four things: conversion of the content from analog to digital form, if necessary; creation of a file in the format recognized by the streaming media server and player; compression of the file to maximize the richness of the content that can be delivered in real-time given limited bandwidth; and, establishing the bit rate at which the media is to be delivered. Content owners typically choose to encode media at multiple rates so that users with fast connections get as good an experience as possible but users with slow connections can also access the content.
Non-streaming content is standards-based in the sense that the server and client software developed by different vendors, such as Apache server, Microsoft Internet Explorer, Netscape Communicator, and the like, generally work well together. Streaming media, however, usually relies on proprietary server and client software. The server, client, production and encoding tools developed by a streaming software vendor are collectively referred to as a format. Streaming media encoded in a particular format must be served by that format's media server and replayed by that format's client. Streaming media clients are often called players, and typically they exist as plug-ins to Web browsers. Streaming media clients are also often capable of playing standards-based non-streaming media files, such as WAV or AVI.
The three major streaming media formats in use today are: RealNetworks RealSystem G2, Microsoft Windows Media Technologies (“WMT”), and Apple QuickTime. RealSystem G2 handles all media types including audio, video, animation, and still images and text. RealSystem G2 and QuickTime support SMIL, an XML-based language that allows the content provider to time and position media within the player window. To deliver the media in real time Real and QuickTime use RTSP, the Real Time Streaming Protocol. RTSP is an application-level protocol designed to work with lower-level protocols like RTP (Realtime Transport Protocol) and RSVP (Resource Reservation Protocol) to provide a complete streaming service over the Internet. To stream in WMT's Advanced Streaming Format, content providers typically must have Microsoft NT 4 Server installed. WMT does not support SMIL or RTSP but has its own protocol that it calls HTML+Time. Apple QuickTime recently has added the capability to serve streaming media. QuickTime can support a number of formats including VR, 3D, Flash, and MP3.
From a network perspective, traditional approaches to streaming Internet content involve transmitting a streaming signal from a source to a device known as a splitter (or repeater, reflector or mirror), which, in turn, replicates the source signal into multiple signals. Each of the multiple signals is the same, and each is sent on to a different destination. By cascading splitters in a tree-like fashion, a single source stream can be replicated into thousands or more identical copies. In this manner, a large number of viewers on the Internet can receive the same streaming signal simultaneously
It is also known in the art to deliver streaming media (and HTTP-based Web content) using a content delivery network (CDN). A CDN is a network of geographically distributed content delivery nodes that are arranged for efficient delivery of digital content (e.g., Web content, streaming media and applications) on behalf of third party content providers. A request from a requesting end user for given content is directed to a “best” replica, where “best” usually means that the item is served to the client quickly compared to the time it would take to fetch it from the content provider origin server.
Typically, a CDN is implemented as a combination of a content delivery infrastructure, a request-routing mechanism, and a distribution infrastructure. The content delivery infrastructure usually comprises a set of “surrogate” origin servers that are located at strategic locations (e.g., Internet network access points, Internet Points of Presence, and the like) for delivering copies of content to requesting end users. The request-routing mechanism allocates servers in the content delivery infrastructure to requesting clients in a way that, for web content delivery, minimizes a given client's response time and, for streaming media delivery, provides for the highest quality. The distribution infrastructure consists of on-demand or push-based mechanisms that move content from the origin server to the surrogates. An effective CDN serves frequently-accessed content from a surrogate that is optimal for a given requesting client. In a typical CDN, a single service provider operates the request-routers, the surrogates, and the content distributors. In addition, that service provider establishes business relationships with content publishers and acts on behalf of their origin server sites to provide a distributed delivery system. A well-known commercial CDN service that provides web content and media streaming is provided by Akamai Technologies, Inc. of Cambridge, Mass.
CDNs may use content modification to tag content provider content for delivery. Content modification enables a content provider to take direct control over request-routing without the need for specific switching devices or directory services between the requesting clients and the origin server. Typically, content objects are made up of a basic structure that includes references to additional, embedded content objects. Most web pages, for example, consist of an HTML document that contains plain text together with some embedded objects, such as .gif or .jpg images. The embedded objects are referenced using embedded HTML directives. A similar scheme is used for some types of streaming content which, for example, may be embedded within an SMIL document. Embedded HTML or SMIL directives tell the client to fetch embedded objects from the origin server. Using a CDN content modification scheme, a content provider can modify references to embedded objects so that the client is told to fetch an embedded object from the best surrogate (instead of from the origin server).
In operation, when a client makes a request for an object that is being served from the CDN, an optimal or “best” edge-based content server is identified. The client browser then makes a request for the content from that server. When the requested object is not available from the identified server, the object may be retrieved from another CDN content server or, failing that, from the origin server.
A “best” content server for a particular client may not remain that way for a given time period. In the context of streaming content delivery, for example, the notion of “best” can change very quickly given the relatively large size of most streams (and the near-infinite size of all live streams). Thus, in many cases, the “best” server for a given client player receiving a given stream is likely to change before the stream is finished.
It would be desirable to enable a client player to identify a best server dynamically and, when appropriate, to enable the player to selectively switch from one server to another during the process of downloading and outputting a given stream or other large file.